On Mar. 23, 1983, President Reagan directed the country to begin research toward the possibility of having a ballistic missile defense by the turn of the Century. The Strategic Defense Initiative (SDI) is divided into five program elements, including one for directed energy weapon technologies. The neutral particle beam (NPB) is one of these technologies. Neutral particle beams have several inherent properties that make them very attractive for space based applications, in particular, high energy neutral particles propagate in straight lines unaffected by the earth's magnetic field and have a very brief flight time to targets even at extended ranges. In addition, the neutral particles upon interaction with the surface of a target become high energy charge particles which penetrate deeply into the target as they deposit their energy.
Among the critical NPB engineering technologies are the development of high average power high beam qualitiy accelerators; large high quality magnetic optics; neutralization techniques which preserve the beam quality; and techniques for sensing and controlling the direction of the beam of neutral particles.
Interest in space based applications of these neutral particle beams began nearly a decade before President Reagan's directions when experiments, at the Los Alamos Clinton P. Anderson Meson Physics Facility (LAMPF), on the proton linear accelerator showed several orders of magnitude improvement in accelerator performance. Extensive measurements of beam properties at energies of 211 and 500 MeV showed that the energy spread of the beam was better than 0.5% and the emmittance of the beam was better that 0.06 cm-mrad. Also, the LAMPF accelerator has been used to accelerate H.sup.- ions to energies above 100 MeV with their behavior being similar to that for protons. These achievements prompted Knapp and NcNally to write a LANL report entitled "SIPAPU" in which they proposed a satellite-based high energy neutral hydrogen weapon; (see SIPAPU Report LA--5642--MS, Los Alamos National Laboratory, July 1974). Their device is depicted schematically in FIG. 1A, where an intense, high qualitiy beam of H.sup.- ions is generated and accelerated to an energy of approximately 250 MeV. After acceleration, the beam is expanded and passed through final focusing and steering magnets. The diameter of the beam in the accelerator and beam transport sections is measured in mm, but after expansion the diameter of the beam is of the order of a meter. Therefore, the beam area has been increased by a factor of the order of 10.sup.6 and the current density has also been decreased by this same amount. This low current density beam is subsequently neutralized by stripping the weakly bound electron from the H.sup.- ion and the resulting hydrogen beam propagates toward the target unaffected by the earth's magnetic field. Both the system and the target must remain above approximately 250 kilometers during the engagement in order to minimize beam degradation due to collisions with residual gases in the atmosphere. However, this does not preclude the system being used in a pop-up faction where the weapon is rocket borne for use in a fly-by or a fly-alone mode for either discrimination, target kill, or both. In case of a nuclear warhead, these particles are capable of heating the nuclear material by fission processes, neutron generation, and ionization. For nonnuclear heavy targets, heating is produced by ionization, possibly producing kill by thermal initiation of the weapon's high explosive. Also, the response of targets to the high energy neutral particle beam is different for lightweight decoys and massive ICBMs which allows these beams to be utilized in a discrimination role where small kinetic kill vehicles, similar to those used as anti-satellite weapons, are used to destroy the ICBMs once they have been identified or if the target is not too far away the NPB may be used to also destroy the identified massive targets.
Improvements in the state-of-the-art for intense high quality (high brightness) negative ion sources and lightweight efficient accelerators have been made. However, additional improvements are needed, and improvements in the state-of-the-art for compact lightweight power systems and for high current neutralizer techniques without excessive scattering are necessary before a device like this can be considered viable. Also, methods for neutral beam detection, signatures for closed loop tracking, for kill assessment, and techniques for rapidly steering the beam over larger angles are also needed. But, even though there are many practical issues to be considered, there does not appear, in principle, to be any inherent limitations that deem the device inviable. Many of the practical issues have been overcome and others are being addressed by the SDIO/BMDATC Neutral Particle Beam program. Among these are solutions for the neutralization of the H.sup.- ion beam and beam sensing techniques.
After the H.sup.- beam has been accelerated, expanded, aimed, and focused on the target, it must be neutralized and the neutral beams direction must be sensed and controlled so that it remains on the target. The technique used for sensing the neutral particle beam can depend on how the beam is neutralized. Neutralization can be accomplished by a number of techniques. For example, photodetachment, plasma, or gas stripping have been considered. Photodetachment causes less degradation in beam quality and can result in the largest fraction of the negative ion beam converted to a neutral beam. Unfortunately, extremely high energy CW lasers at wavelengths where these power levels are not currently available are required for this purpose, and even if they become available, they would probably be as large or as expensive and require as much prime power as the rest of the system. Since open ended plasma strippers with quiescent plasmas might cause less degradation in beam quality and neutralize a slightly large fraction of the negative ion beam than a gas stripper, they also have been studied. But, the power requirement for the plasma stripper alone is equal to or greater than that for the rest of the system. Also, it is problematical that a sufficiently quiescent plasma could be produced. Therefore, considerable work both theoretical and experimental has been devoted and is being devoted to the development of a gas stripper. The important results of this work is summarized in FIG. 1B where the fraction of the initial beam which survives as H.sup.-, the fraction which is stripped to H.sup.0, and the fraction which is stripped to H.sup.+ is given as a function of the stripper thickness. Also, shown is the component of the H.sup.0 beam which has not been elastically scattered (i.e., the useable part of the H.sup.0 beam for targets at long ranges) and the component of the H.sup.0 beam which has been elastically scattered (this is useful for beam sensing purposes). However, the gas neutralizer allows gases to escape out the ends and this has serious adverse systems implications. These implications are eliminated by the teachings of Roberts, Havard, and Wilkinson in U.S. patent application Ser. No. 397,371 titled "Solid Striper for a Space Based Neutral Particle Beam System:" and by Roberts, Edlin, and Strickland in AMPC 4358 "Supported Thin Foil Stripper and Simple Non-Obstructing Power Meter for a Space Based Neutral Particle Beam System." The data in FIG. 1B also applies to these strippers and that part of the neutral beam which has been elastically scattered and is in the metastable 2S state is available for beam sensing by the techniques of laser resonant fluorescence (LRF).
The technique of laser resonant fluorescence for sensing the direction of a neutral hydrogen particle beam was first published by G. Rohringer in 1977. The direction of the neutral hydrogen beam is sensed by shining a laser beam on the particle beam and observing the angle between the laser beam and the particle beam which caused a fluorescence signal to be generated (G. Rohringer "Particle Beam Diagnostics by Resonant Scattering," General Research Corp. # GRC-1-783 (1977)). To accomplish this, either a very high frequency ultraviolet laser was needed, or a significant fraction of the hydrogen atoms in the beam had to leave the stripper in the excited 2S metastable electronic state. Early predictions had suggested that there were not enough of these excited atoms, but the LANL 192 experiment on gas stripping indicated that there is approximately 8 percent in the 2S state, which is enough for beam sensing. Charles Starke Draper Laboratories performed a study to determine the theoretical limits on the accuracy that might be obtained by the laser resonant fluorescence technique. This effort included defining the mission requirements for the acquisition, pointing, and tracking and estimating the accuracy of the LRF technique for both sensing the direction of the beam and overcoming jitter. Also, an optical reference gyro could be used to align the LRF lasers and the target line of sight. An LRF sensing and control algorithm has been developed where emphasis is placed on using sub-beam sampling to control the mean direction of the beam, the focusing errors and beam abberations. Also, an autoalignment system was designed and control algorithms for the magnets in the beam line were developed. Finally an experimental verification of the LRF technique was performed (Moses, D., et. al., "The Angular Distribution of Neutral Hydrogen Following Collisional Electron Detachment from H.sup.-," Los Alamos National Laboratory, (1981)) which validated the Geoni-Wright theoretical predictions for the angular divergence of the beam atoms lift in the 2S electronic state by a gas stripper (Genoi, T. C., and Wright, L. A., J. Phys. B, 13, 461, 1980)). However these experiments were performed on a Van de Graff accelerator at low energy and the accuracy obtainable by the LRF techniques could not be checked.
The frequency, .upsilon..sub.b, seen by the atoms in the beam when the laser frequency in the lab frame is .upsilon..sub.L is given by EQU .upsilon..sub.b =.upsilon..sub.L .gamma.(1-.beta.cos .theta.) (1)
where .gamma.=1/(1-.beta..sup.2).sup.1/2, .beta.=v/c and V is the velocity of the particles in the beam, C is the velocity of light in vacuum, and 0 is the angle between the laser beam and the neutral particle beam. The sensitivity of .upsilon..sub.b to small changes in the angel .theta. is given by ##EQU1## the sensitivity of .upsilon..sub.b to changes in .beta. is given by ##EQU2## where it may be seen that at the angle EQU .theta..sub.DF =cos .sup.-1 .beta.we have (4)
(.delta..upsilon..sub.b)=0 Thus, at this angle (known as the Doppler free angle) .upsilon..sub.b is insensitive to small changes is .beta. which results from the residual momentum spread in the beam. The sensitivity of .upsilon..sub.b to small drifts in the laser frequency is simply EQU .delta..upsilon..sub.b =.gamma.(1-.beta.cos .theta.).delta..upsilon..sub.L ( 5)
or EQU (.delta..upsilon..sub.b)/.upsilon..sub.b =(.delta..upsilon..sub.L)/.upsilon..sub.L ( 6)
Therefore, we can work at or near the doppler free angle as determined from equation 4 for the beam energy of the system under consideration by choosing the laser frequency .upsilon..sub.L so that .upsilon..sub.b as given by equation (1) is near the maximum of the cross section for exciting H(2S) to H(3P) in the beam frame, and thus producing the desired fluorescence.
Note that the direction of the beam is determined from measured values of .theta., .beta., and .upsilon..sub.L, the above method will give the most accurate results providing .upsilon..sub.L, is accurately known and does not change. Therefore the laser will have to be frequency stabilized. But even frequency stabilized lasers will experience frequency drifts when long term stability is attempted. Thus a need exists to compensate for this frequency drift so that it does not show up as a measured drift in the direction of the neutral beam. Therefore, and object of this disclosure is to provide a device which automatically compensates for small drifts in .upsilon..sub.L.